I don't know who is to "blame" if anyone at all. But honestly, I have a 6yo, a 4yo with autism who frequently runs off and has no concept of danger and a love for animals, a 3yo who is a runner and a smart arsed little 2yo. So I definitely don't blame the mother as I'm sure you can probably all guess I may have lost a child or two (I've been the "we have a boy in a red jumper at the security desk" over the loud speaker type of mum...)
I also don't particularly want to judge the zoo and their enclosure as in my experience, otherwise suitable fences don't always work. I have actually watched my son with autism, when he was 2.5yo, climb up and over a regulation pool fence. I'm terrified of what he would be able to do now at 4yo.
I too hate the implication that making a mistake can suddenly deem you as the worst parent in the world. I'm sure she feels it anyway. She doesn't need a bunch of anonymous commenters telling her it.
I do feel sad that the animal had to die. I can see why and agree with it going the way it did but I don't have to like it. However, I am always going to put the human child first. If it's their fault (or the parents) I don't expect them to escape reprimand and I don't always think an animal should be punished for its actions (I had a bad dog bite as a child and refused to go to hospital until they promised it would live) but if it came down to a choice between human or animal being saved, I will choose human. Especially a child.